Sodium thiosulphate
10cm3 cylinder
100cm3 cylinder
250ml conical flask
Stopwatch
Thermometer
Piece of paper with cross on
Goggles
Method
1. Mark a cross, (with a fine pen), on a piece of paper. Place on the table.
2. Place a 250ml conical flask on to the cross, (see the cross through the glass).
3. Measure out 50cm3 of sodium thiosulphate in to a 100cm3 cylinder, and pour into the flask.
4. Measure out 5cm3 of hydrochloric acid in a 10cm3 cylinder.
5. Have a stopwatch at hand.
6. Quickly pour the acid into the flask, and at the same time, or immediately after, start the stopwatch.
7. Once you can no longer see the cross through the flask, stop the stopwatch.
8. Record the time taken.
9. Repeat these steps four more times, using 10cm3, 20cm3, 30cm3and 40cm3 of water, lowering the volume of the sodium thiosulphate by the same amount, to make 50cm3 of solution.
10. Record all the results and do repeats to check.
Rates of reaction and collision theory
Temperature will affect my reaction, when two chemicals react; their molecules have to collide with each other with sufficient energy for the reaction to take place. This is collision theory. The two molecules will only react if they have enough energy. By heating the mixture, i will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster.
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. Although you keep the temperature constant, kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
The reaction is between two solutions, i might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference. What i need to do is fix the concentration of one of the reactants to excess. Now i can increase the concentration of the other solution to produce an increase in the rate of the reaction. When a catalyst undergoes collision with the reactant molecules, less energy is required for the chemical change to take place, and so more collisions have sufficient energy for reaction to occur. The reaction rate therefore increases.
I predict that the more concentrated the solution of sodium thiosulphate the faster the reaction will be. I think that if I double the concentration of sodium thiosulphate then I will double the rate of reaction as well. When I do the experiment I will always use the same total volume of liquids in the flask. I will always use 5cm3 of hydrochloric acid, but will also use different volumes of the sodium thiosulphate to get different concentrations and results. I have only been given one concentration of sodium thiosulphate but I want at least six different concentrations. This means I need to dilute it with water for some of the readings on my table that I have made below.
In this plan I will have 60 cm3 of sodium thiosulphate solution, although its concentration will change in a controlled way. I will have to measure out the volume of each liquid I need to separate measuring cylinders before the reaction. I will then put the acid and water in to the flask before the experiment starts then I will add the sodium thiosulphate solution, then when I put this in I will start the stopwatch and swirl the flask so the solutions mix. When the reaction is over I will wash the flask out because there might be residue of the experiment before in the flask we are using and to get fair results this cannot happen. I will repeat this process for each experiment. I will do my experiments at room temperature because temperature can effect the reaction.
